Previously we observed that resistance to infection of the lungs by respiratory syncytial virus (RSV) was transferred from immune cotton rats to their non-immune parabiotic partners. Passive transfer experiments suggested that serum, rather than cells, mediated this protective effect, as cotton rats pretreated with either cotton rat or human serum containing RS virus antibodies were completely resistant to pulmonary infection. Duplication of this effect using purified human IgG identified serum antibody as the effector. Human antiserum (or purified IgG) administered to cotton rats after RSV infection resulted in complete or near-complete clearance of pulmonary virus, suggesting a potential technique for specific treatment of established RSV disease. Potentiated disease that developed during RS virus infection of individuals previously given a formalin-killed RSV vaccine (1966-1967) remains unexplained. Using the cotton rat we have developed an experimental model for examining vaccine-induced potentiation. Early results suggest an Arthus-type reaction. RSV was inoculated into a second strain of cotton rats, in which it produced overt signs of disease for the first time. Efforts are under way to establish a colony of this strain of cotton rat.